A common system for pumping large volumes of fluid from a hydrocarbon well employs an electrical submersible pump assembly. The pump assembly includes a centrifugal pump and a down hole electrical motor. The pump is made up of a large number of pump stages, each pump stage having an impeller and a diffuser. The impeller rotates and imparts velocity to the well fluid while the diffuser converts the kinetic energy to pressure.
Pumps of this type efficiently pump liquids, but many hydrocarbon wells produce both liquid and gas. Efficiently pumping two-phase fluids with a centrifugal pump is difficult if the density difference between the two phases is significant. The impeller stages of a centrifugal pump increase the pressure by imparting velocity to the fluid. The pressure that is created is a function of the density of the fluid. For example, if the liquid components of the well fluid had a density 100 times greater than the gaseous components, the gas would require ten times more velocity to achieve the same pressure as the liquid. Oil has approximately 100 times the density of natural gas at approximately 150 psi. An impeller of a centrifugal pump cannot accomplish the differences in velocity, resulting in the lighter fluid gathering in pockets near the center of rotation. These pockets have great difficulty in moving into the area of high pressure, and therefore grow larger, blocking the flow area and reducing the pressure creation ability of the pump stage until it has been reduced to the point where the gas can move.
One approach to solve the problem of gas content in hydrocarbon well fluid is to utilize a gas separator. The gas separator locates below the pump and separates gas from the liquid, typically by a forced vortex. The forced vortex forces the heavier components to the outer portions of the gas separator housing, leaving the lighter components near the axis of rotation. The heavier components have a much higher velocity than the lighter components. A crossover at the upper end of the gas separator guides the heavier fluid components back into the central area and into the intake of the pump. The lighter fluid components are diverted outward from the gas separator into the casing.